Noise cancellation headset

ABSTRACT

A headset for acoustic reproduction of an electronic signal which electronic signal is representative of the summation of a desired audio signal and an anti-noise signal. The headset includes a headband, an earcup mounted to the headband, a driver mounted within the earcup which receives and acoustically reproduces the electronic signal, a directional microphone which detects and transduces the acoustical pressure within the earcup cavity, means for generating the anti-noise signal from the microphone signal, and a positioning member for mounting the microphone to the earcup in a position so that the microphone is acoustically coupled to the driver. The microphone is oriented so that its vented or open face is pointing towards the driver. If the driver is provided with an inner dome portion and an outer annulus portion the microphone is positioned by its attachment to a grille plate extending across the driver, the microphone being mounted to the grille plate on a side opposite the driver. The grille plate includes a plurality of apertures, at least one of which is positioned proximate the center of the driver with the microphone mounted across that aperture so that the microphone is acoustically coupled to the dome portion and acoustically isolated from the outer annulus portion.

FIELD OF THE INVENTION

The present invention relates generally to the field of headsets, andmore particularly to headsets which provide for the cancellation ofnoise during operation.

BACKGROUND OF THE INVENTION

Voice communication between people or with one person located in anenvironment having significant background or ambient noise can bedifficult, and burdensome--in some cases even dangerous ifmiscommunication occurs. Additionally, working in such an environmentcan become impossible in some circumstances if there is no shielding ofthe worker from the noise. As a result of this need to communicate or toshield workers from excessive ambient noise, various devices have beendeveloped.

It has been stated that a typical approach for noise attenuation orshielding in the workplace is to provide workers with headsets havinghigh mass, large internal volume and a spring support that exerts heavypressure upon the head, i.e. forces the earcup against the head of thewearer. The high mass and heavy pressure operate to create a seal withthe wearer's head which in turn serves to attenuate low frequency noisewhile the large internal volume provides so-called high frequency rolloff. The problem with such a passive noise attenuation approach was saidto be the discomfort associated with the wearing of such devices.

Other previously described headsets utilized an active noise attenuationapproach, wherein a cancellation or anti-noise signal is generated andadded to the signal being applied to the headset. Upon beingacoustically reproduced, the anti-noise component tends to cancel thebackground or ambient noise within the region around the ear. Such prioractive attenuation techniques are generally of two types, so-called openloop headset systems and closed loop headset systems.

An example of a headset system incorporating open loop components can befound in U.K. Patent Application No. 2,104,754 A - Chaplin, publishedMar. 9, 1983, wherein a sensor is used to detect a repetitive noise rateor frequency, which in turn is used by a waveform generator to generatea cancellation signal. The closed loop components depicted in this U.K.application will be discussed more fully below. The problems associatedwith open loop headset systems are first, they may be limited to onlycancelling certain background noise and second, it is difficult to takepassive headset attenuation into account. Closed loop headset systems,however, do account for passive attenuation and tend to cancel allbackground noise.

An early closed loop headset system can be found in Olson, H.,Acoustical Engineering, Van Nostrand, New York 1957, pps. 415-418.Subsection C, Headphone-Type Noise Reducer, describes a headset whereina microphone has been placed in an earcup closely adjacent to adiaphragm for the purpose of providing noise reduction. The microphonesenses the pressure in the cavity formed by the earcup on the ear andprovides a feedback signal representative of such pressure. It isunderstood that the pressure in the cavity is reflective of not only theacoustic reproductions of the driver but also external noise which haspenetrated the earcup. The microphone signal can be used to cancel ornull the external noise by shifting the phase of the signal.Consequently, when acoustically reproduced, the noise component of themicrophone signal tends to acoustically cancel external noise present inthe cavity.

Another closed loop system can be found in the report AD-A009 274entitled A STUDY OF PROPOSED EAR PROTECTION DEVICES FOR LOW FREQUENCYNOISE ATTENUATION by P. M. Dallosta dated April, 1975 at pages 86, 87and 117-119. The report depicts a microphone within an earcup for use ina cancellation circuit. The cancellation circuit is shown at pages117-119 to sum the communications signal with the microphone signalafter the microphone signal has been provided proper gain and phaseshift for optimum cancellation. The resultant summed signal is amplifiedand supplied to the headset.

As indicated above, U.K. Patent Application No. 2,104,745 also disclosesclosed loop components. Specifically, a microphone is said to beprovided closely adjacent a speaker in a headset earphone. The signalfrom the microphone is fed back to an "adaptive means" which in turnutilizes the microphone signal and the previously described open loopsignal to generate a cancellation signal. When the cancellation signalis reproduced by the speaker, it is said that the noise field is nulledexcept for certain desired sounds. It is also indicated that themicrophone signal could be used in a direct feed-back system forattenuating mid-band frequencies.

U.S. Pat. No. 4,455,675 - Bose et al. discloses a closed loop headsetsystem wherein a microphone is mounted coaxially with a driver in aheadphone. The open or vented region of the microphone is directed awayfrom the driver and towards the ear canal. The signal generated by themicrophone is combined with the signal desired to be reproduced by thedriver. Prior to providing the combined signal to the driver, thecombined signal is said to be processed by passing it through acompressor to limit the level of high level signals and thereafter it isapplied to a compensator to ensure that the open loop gain meets theso-called Nyquist stability criteria to prevent system oscillation.

Somewhat related U.S. Pat. No. 4,644,581 - Sapiejewski discloses aclosed loop headset system similar in design to that shown in U.S. Pat.No. 4,455,675 except for two (2) features. Damping material has beenpositioned to cover the headphone cavity which contains the driver.Also, instead of orienting the microphone coaxially with the driver anddirecting the open or vented microphone face towards the ear canal, themicrophone is located off-set from the driver axis and oriented so thatits diaphragm is perpendicular to a plane containing the driverdiaphragm, i.e. the open or vented face is directed perpendicular to thedriver axis. The perpendicular orientation is said to result inincreased bandwidth of the closed loop and the off-set location is saidto reduce peaks in frequency response at the high end.

The problem which remains despite these prior open and closed loopheadset systems, is that phase lag, particularly at high frequencies,has not been minimized. Such phase lag can occur as a result of severalfactors, one of which is the propagation delay associated with thedistance between the microphone and driver.

SUMMARY OF THE INVENTION

The above described problems are overcome by a headset for acousticproduction of an electronic signal which electronic signal isrepresentative of either an anti-noise signal or the summation of adesired audio signal and an anti-noise signal. The headset includes aheadband, an earcup mounted to the headband, a driver mounted within theearcup which receives and acoustically reproduces the electronic signal,a microphone which detects and transduces a total acoustic pressurewithin the earcup, means for generating the anti-noise signal from themicrophone signal, and a positioning member for mounting the microphoneto the earcup in a position so that the microphone is acousticallyclosely coupled to the driver. The microphone is oriented so that itsvented or open face is pointing towards the driver. Preferably themicrophone is mounted so that its diaphragm is as close as possible tothe speaker diaphragm, and preferably as close as possible to thatportion of the speaker with the best HF performance (so as to result inlow phase lag). Preferably, also, the position of the microphoneisolates it as much as possible from out of phase acoustic reflectionsand mechanical standing waves. In a Particularly preferred embodiment,if the driver is provided with an inner dome portion and an outerannulus portion the microphone is positioned by its attachment to agrille plate extending across the driver, and the microphone is mountedto the grille plate on a side opposite the driver. The grille plateincludes a plurality of apertures wherein at least one aperture ispositioned proximate the center of the driver; the microphone is mountedacross that aperture so that the microphone is acoustically coupled tothe dome portion and acoustically substantially isolated from the outerannulus portion.

These and other objects and advantages of the invention will become moreapparent from the following detailed description when taken inconjunction with the following drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view of a headset according to the presentinvention;

FIG. 2 is a section view of the right earcup of the headset shown inFIG. 1;

FIG. 3 is an enlarged view of a portion of the earcup shown in FIG. 2;and

FIG. 4 is a block diagram of the circuit depicted generally in FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As will be more completely described with regard to the figures, thepresent invention is embodied in a new and novel headset system shown inFIG. 1 and generally designated 10. It will be noted that only the rightside of the headset is shown and described, since the right and leftside are mirror images of each other.

Headset 10 is shown to include headband 12, earcup 14 which is mountedin any known manner to headband 12, driver 16 mounted within earcup 14which receives and acoustically reproduces desired electronic signals,and microphone 18 having a vented face 20. Microphone 18 generates asignal representative of the acoustic reproductions from driver 16 andbackground or ambient noise which enters the cavity formed betweenearcup 14 and the wearer. Microphone 18 is attached to grille plate 22by any appropriate means so that microphone 18 is centrally positionedover driver 16 and wherein vented face 20 is oriented to point towardsdriver 16. As will become apparent in relation to FIGS. 2 and 3,microphone 18 is positioned in relation to driver 16 so that it isacoustically closely coupled thereto. Grille plate 22 is attached toearcup 14 and extends across the front face of driver 16. In thepreferred embodiment, microphone 18 is a standard electret microphonesuch as a Knowles 1759.

As indicated above, the signal generated by microphone 18 is provided toelectronic circuit 24, at input 26. Electronic circuit 24 processes thedesired audio signal provided at input 28, and generates a modifiedsignal for acoustical reproduction by the headset. The modified signalis generated by summing the original audio signal (if any) and ananti-noise signal that is generated based on the signal obtained frommicrophone 18. The modified signal is thereafter provided to driver 16.Since the anti-noise signal is added to the original audio signal, aportion of the modified signal reproduced by driver 16 will have theeffect or tendency to cancel or null background noise in the earcupcavity; also, if the frequency response of the microphone is relativelylinear the microphone may detect distortion caused by the speaker asnoise and effectively reduce or cancel such distortion as well. As willbe more particularly described in relation to FIG. 4, the modifiedsignal may be passed through a plurality of cascaded filters foraccentuating a desired frequency range prior to being provided to driver16.

Referring now to FIG. 2, earcup 14 is shown to have an ear cushion 30secured to its outer edge. As will be understood, the force resultingfrom earcup 14 being pressed against the wearer's head will causecushion 30 to conform to the shape of the head thereby creating anacoustical seal, contributing some passive noise attenuation capabilityto the headset. Many of such cushions are known and any would besuitable for the purposes of the present invention.

Driver 16 is shown to be generally centrally mounted within earcup 14 bymounting bracket 32. In the preferred embodiment, the driver 16 may besecured by suitable means such as adhesives into bracket 32; alternatelythey could be integrally formed. In either case, a secure attachment ofthe bracket and driver optimizes the so-called electrical-to-acousticaltransfer function and prevents front to back acoustic leaks which couldlimit low frequency response. In order to control rear loading a hole 34is provided in bracket 32 providing fluid communication between bracketcavity 36 formed between driver 16 and bracket 32 and earcup cavity 38formed between earcup 14 and bracket 32. Such rear loading is furthercontrolled through the attachment of acoustical cloth 40 over hole 34.Foam overlay 42 is positioned in the open end of earcup 14 to providefurther control of the overall phase characteristic of the headset. Foamoverlay 42 damps acoustic resonant frequencies above 1 kHz, reducing themagnitude fluctuations, keeping the phase from exhibiting a phase dropat approximately 1.5 kHz, and reducing undesirable high frequency phasevariations.

Referring now to FIG. 3, microphone 18 is shown to be generallycoaxially mounted with respect to driver 16. Microphone 18 is mounted insuch a position by its attachment to the center of grille plate 22.Grille plate 22 is provided with a number of apertures 44 spaced aboutthe center of grille plate 22 in any desired pattern to optimizecoupling, low delay from electrical input to mic output, and minimumacoustic/mechanical standing waves. In a preferred embodiment the grilleplate 22 has a central aperture 46 which is positioned directly over thecenter of driver 16 when grille plate 22 is mounted within earcup 14.Microphone 18 is mounted across aperture 46 so that its vented or openface 20 is Pointing through the central aperture 46. Microphone 18 isshown to be held in place by a bead of adhesive 48.

As shown in FIG. 3, driver 16 includes a coil portion 50, a central domeportion 52 and an outer annulus portion 54. The distance betweenmicrophone 18 and driver 16 is desirably less than one-quarterinch--preferably less than about one eighth of an inch. The larger theseparation between microphone 18 and driver 20 the larger the phase lagresulting in the headset system--the separation must be small incomparison to the wavelengths corresponding to frequencies of interestto avoid excessive Phase lag. Orienting microphone 18 in the describedfashion is such that microphone 18 becomes closely acoustically coupledto driver 16, resulting in a minimum phase lag. By interposing grilleplate 22 between driver 16 and microphone 18 so that microphone 18 ispointing through central aperture 46, microphone 18 becomes closelycoupled to dome portion 52 and substantially acoustically isolated fromouter annulus portion 54.

The importance of acoustically coupling microphone 18 to driver 16 andmore particularly to dome portion 52 lies in that such coupling causes asharp 60° to 90° phase lead for signal frequencies occurring atapproximately 1.5 kHz. This phase lead results in the headset systemhaving relatively flat phase characteristics from about 1 kHz to about2.3 kHz. Consequently, the phase compensation electronics can bedesigned relative to the dominant headset resonance which will cause arapid drop in the phase characteristics. In relation to the previouslydescribed headset system such rapid drop occurs at some point betweenapproximately 2.5 kHz and 3 kHz.

By mounting microphone 18 to grille plate 22 on a side opposite driver16, microphone 18 is acoustically substantially isolated from outerannulus portion 54. The importance of isolation from outer portion 54lies in that the outer portion of the driver diaphragm will break upinto higher modes of vibration and give rise to large phase shifts atfrequencies above 1 kHz, i.e. phase shifts of about 90° to 180°.

As indicated above, the electronic signal provided to driver 16 isgenerated by summing the anti-noise signal (derived from the microphonesignal) with the desired audio signal (if any--the noise cancellationoperates whether or not any such audio signal is provided). As shown inFIG. 4, the desired audio signal from a communications radio 56 orsimilar source is provided to one of the positive inputs of summingcircuit 58. The signal detected at the microphone 18 is passed throughmic amp 60, passed to a noise cancellation signal processing unit 61(described below), and then inverted to create the anti-noise signalwhich is applied to the other positive input of summing circuit 58(alternately, the signal may merely be inverted by providing it to anegative input of the summing circuit).

The amplifier 60 and equivalent components in the communications radio56 may be adjusted with respect to one another to set the relativelevels of the desired audio and anti-noise signals. A standardmicrophone 63 may also be provided on the headset to receive andtransmit speech of the headset wearer to the communications radio 56.

The total acoustic pressure transduced by the earcup microphone 18 is acombination of the acoustic output of the driver 16 and external ambientnoise that has penetrated into the cavity formed between earcup 14 andthe wearer. (In the preferred embodiment the headset is a conventionalpassive noise attenuating headset; such headsets do an acceptable job atattenuating higher frequency noise, but are not as effective atattenuating some strong components of, e.g., aircraft motor and windnoise that occurs at frequencies below about 300 Hz.) The electricsignal from the microphone 18 is amplified by mic amp 60 and provided tonoise cancellation signal processing unit 61.

The resulting noise signal is then inverted to provide an anti-noisesignal that may be amplified and provided to the driver to cancel thedetected noise in the headset. This cancellation is effective whether ornot an additional radio communication signal (or similar signal) is alsoprovided to the driver.

The aggregate signal transfer characteristic of 60, 61 and 68, asdepicted in circuit 24, can be manipulated by choosing the transfercharacteristic of unit 61 to provide phase/magnitude compensation whichmaintain the closed loop characteristics stable with an as large aspossible open loop gain in the audio frequency range. The compensationmust deal with both low and high frequency instabilities. To this endthe modified audio signal, generated by the summation with theanti-noise signal, may be further processed before it is amplified andpresented to driver 16. The modified audio signal may be processed byits passage through a plurality of cascaded filters designed toaccentuate a desired frequency range while minimizing undesirable phasecharacteristics. The exact parameter values of each filter are dependentupon the particular driver, microphone and other acoustic elementschosen for a given headset system.

As shown in FIG. 4, the series of cascaded filters may include a highpass filter 62 for filtering out those components of the modified audiosignal having frequencies below a first frequency, and a low pass filter64 for filtering out those components of the modified audio signalhaving frequencies above a second frequency. In the preferred embodimentthe first frequency is in a range extending from 17 to 70 Hz and thesecond frequency is also in a range extending from 150 Hz to 8 kHz. Itshould be understood that in filter 62 frequency components of themodified audio signal falling below 17 Hz will be attenuated by a fixedamount and that attenuation of the modified audio signal will decreaselinearly for frequency components falling between 17 Hz to 70 Hz atwhich no attenuation takes place. Similarly, in filter 64 frequencycomponents of the modified audio signal falling above 8 kHz will beattenuated by a fixed amount and that attenuation of the modified audiosignal increases linearly for frequency components falling between 150Hz and 8 kHz at which maximum attenuation takes place. The 8 kHz value,a so-called pole, was chosen for the preferred embodiment to minimizephase lag around the frequency of 2.5 kHz.

The series of filters may also include a mid-range filter 66 forproviding additional attenuation to those components of the modifiedaudio signal having frequencies falling between a third and fourthfrequency. In the preferred embodiment, the third frequency is 600 Hzand the fourth frequency is 1200 Hz. The centering of this range at 850Hz provides a second order roll-off in a range where some phase lag canbe tolerated.

The filtered modified audio signal is then provided to power amplifier68 for amplification to a level appropriate for driver 16.

As a result of the above described invention, background or ambientnoise reduction within the earcup as a function of frequency ispredictable from the open loop frequency response of the headset system.Maximum noise reduction will occur at frequencies from about 30 Hz toabout 200 Hz at values of approximately 15 to 28 dB. Above and below the30 to 200 Hz frequency range, noise reduction falls off according tocharacteristics determined by the selection of filter parameters.Conventional passive noise reduction built into the headset, however,provides fairly effective noise reduction at higher frequencies, makingthe headset quite effective in achieving the desired overall reductionof noise, particularly for aircraft applications.

While the invention has been described and illustrated with reference tospecific embodiments, those skilled in the art will recognize thatmodification and variations may be made without departing from theprinciples of the invention as described herein above and set forth inthe following claims.

What is claimed is:
 1. A headset for acoustic reproduction of anelectronic anti-noise signal, the headset comprising:a headband; a cup,mounted to the headband; a driver mounted within the cup, which receivesand acoustically reproduces the electronic signal; a microphone, havinga directional sensing surface which detects and transduces acousticpressure within the cup to a corresponding microphone electrical signal;means for generating the anti-noise signal from the microphone signal;and portioning means for mounting the microphone to the cup and forcentering the microphone over the driver with the directional sensingsurface oriented substantially directly toward the driver thepositioning means comprising a grille plate mounted to the cup acrossthe driver, the microphone being mounted to the grille plate.
 2. Theheadset of claim 1, wherein the grille plate includes a plurality ofapertures therethrough, at least one aperture being positioned proximatethe center of the driver and the microphone being mounted across suchaperture.
 3. The headset of claim 2, wherein the microphone is mountedso that the directional sensing surface is pointing through the at leastone aperture toward the driver.
 4. The headset of claim 3, wherein thedistance separating the microphone from the driver is less than aboutone quarter (1/4) of an inch.
 5. The headset of claim 2, wherein theapertures are round.
 6. The headset of claim 1, further comprisingmounting bracket mounted in the cup, the driver being mounted on themounting bracket.
 7. The headset of claim 6, wherein the mountingbracket defines a bracket cavity within the cup and wherein the driveris mounted within the bracket cavity.
 8. The headset of claim 7, whereinthe mounting bracket is secured within the cup thereby defining anearcup cavity so that the volume of the earcup cavity is sealed from theair outside of the cup.
 9. The headset of claim 1, further comprising afoam layer attached to the cup covering the driver and the microphone.10. The headset of claim 1, further comprising a soft cushion mounted onthe outer edge of the cup so that when the cup is pressed on the head ofa user, the cushion is between the cup and the user's head.
 11. Theheadset of claim 1, further comprising a second cup, mounted to theheadband; a second driver, mounted within the second cup, which receivesand acoustically reproduces the electronic signal; a second microphonehaving a directional sensing surface,; and second positioning means formounting the second microphone to the second cup and for centering thesecond microphone over the second driver, wherein the directionalsensing surface is oriented directly toward the second driver.
 12. Theheadset of claim 1, including means for summing a desired audiocommunications signal with the anti-noise signal before providing theanti-noise signal to the driver so that the sum of the desired audiosignal and the anti-noise signal and the anti-noise signal is providedto the driver.
 13. The headset of claim 12, further comprising aplurality of cascaded filters for accentuating a desired frequencyrange.
 14. The headset of claim 13, wherein the cascaded filtersincludes a high pass filter for filtering out those components of theelectronic signal having frequencies below a first frequency, and a lowpass filter for filtering out those components of the electronic signalhaving frequencies above a second frequency.
 15. The headset of claim14, further comprising a mid-range filter for providing additionalattenuation to those components of the electronic signal havingfrequencies falling between a third and fourth frequency.
 16. Theheadset of claim 15, wherein the first frequency is 17 Hz, the secondfrequency is 8 kHz, the third frequency is 600 Hz and the forthfrequency is 1200 Hz.
 17. The headset of claim 7, wherein the driver issealingly mounted within the mounting bracket.
 18. A headset foracoustic reproduction of an electronic anti-noise signal, the headsetcomprising:a headband; a cup, mounted to the headband; a mountingbracket mounted in the cup, the mounting bracket defining a bracketcavity within the cup on one side of the bracket, the mounting bracketbeing secured within the cup to define with the cup an earcup cavity sothat the volume of the earcup cavity is sealed from air outside of thecup; the mounting bracket further containing a bore, thereby providingfluid communication between the earcup cavity and the bracket cavity; adriver, which receives and acoustically reproduces the electronicsignal, mounted on the mounting bracket with in the bracket cavity; amicrophone, having a directional sensing surface which detects andtransduces acoustic pressure within the cup to a correspondingmicrophone electrical signal; means for generating the anti-noise signalfor the microphone signal; positioning means for mounting the microphoneto the cup and for centering the microphone over the driver with thedirectional sensing surface oriented substantially directly toward thedriver.
 19. The headset of claim 18, further comprising a cloth attachedto the mounting bracket to cover the bore.
 20. A headset for acousticreproduction of an electronic signal which electronic signal isrepresentative of the summation of an audio signal and an anti-noisesignal, the headset comprising:a headband; a cup, mounted to theheadband; a driver, mounted within the cup, which receives andacoustically reproduces the electronic signal; a directional microphonehaving a vented face; means for generating the anti-noise signal fromthe microphone signal; a grille plate mounted to the cup across thedriver, the grille plate including a plurality of aperturestherethrough, at least one such aperture being positioned proximate thecenter of the driver, the microphone being mounted to the grille platewith the vented face of the microphone being mounted across such centralaperture with the vented face pointing therethrough orientedsubstantially directly toward the driver.
 21. A headset for acousticreproduction of an electronic signal which electronic signal isrepresentative of the summation of an audio signal and an anti-noisesignal, the headset comprising:a headband; a cup, mounted to theheadband; a driver, mounted within the cup, which receives andacoustically reproduces the electronic signal, the driver having aninner dome portion and an outer annulus portion; a directionalmicrophone having a vented face; means for generating the anti-noisesignal from the microphone signal; and positioning means for mountingthe microphone to the cup so that the microphone is acoustically coupledto the dome portion and isolated from the outer annulus portion, thepositioning means comprising a grille plate mounted to the cup acrossthe driver, the microphone being mounted to the grille plate on a sideopposite the driver.
 22. The headset of claim 21, wherein the grilleplate includes a plurality of apertures therethrough, wherein at leastone aperture is positioned proximate the center of the driver andwherein the microphone is mounted across the at least one aperture. 23.The headset of claim 22, wherein the microphone is mounted so that thevented face is pointing through the at least one aperture.